Fiber and semiconductor lasers and diode-pumped solid-state (DPSS) pulsed lasers with output power in the range of several Watts to tens of Watts are applied primarily in laser micromachining in the field of electronic device manufacturing. Micromachining applications require a high pulse repetition frequency (PRF), corresponding to laser pulse durations ranging from nanoseconds to picoseconds, and even femtoseconds. Typical laser output wavelengths range from infrared to ultraviolet. The performance of traditional solid-state lasers that rely on a simple master oscillator is falling behind the overall pace of laser system technology development, primarily because of limited pulse repetition rate and power scaling by a single oscillator.
As is well known to those skilled persons, progress in power scaling of a TEM00 laser mode has been limited by the formation of aberrated thermal lenses within the active lasing medium. A thermal lens is mainly caused by a temperature gradient in a laser crystal and results in a distortion of the index of refraction of the crystal in response to non-uniform pump power. Peng, Xu, and Asundi, “Power Scaling of Diode-Pumped Nd:YVO4 Lasers,” IEEE-Quantum Electronics, Vol. 38, No. 9, 2002, demonstrate that maximum pump power varies inversely with doping concentration, and that the pump power increases to only 40 W for a 0.3% doped vanadate crystal using an 808 nm pump wavelength and a 0.8 mm diameter pump spot size. FIG. 1 is a graph showing maximum pump power as a function of doping concentration for an 808 nm-pumped laser. In addition to thermal lens formation, the maximum incident pump power is restricted by thermal fracture of the laser crystal. To date, the highest output power achieved for a TEM00 mode narrow bandwidth and linearly polarized beam generated by an end-pumped vanadate laser is less than 30 W, while a power level of about 100 W is desirable. Currently, nanosecond pulsed fiber lasers are limited to generating peak power exceeding 1 kW with a TEM00 mode because of stimulated Brillouin scattering (SBS) and damage issues.
One way to meet the demand for a high power laser source is to use a laser power amplifier. An advantage of laser power amplifiers is that the final power output may be easily scaled to meet a specific requirement for each different application. Laser power amplifiers also may be paired with different seed laser sources to allow flexibility in seed laser design and manufacturing. However, maintaining a high quality beam and stable output in a laser power amplifier remains a technical challenge.
A typical laser power amplifier uses a single-pass configuration, meaning that the seed laser beam passes once through the gain medium. One example is presented in Maik Frede et al., “Fundamental mode, single-frequency laser amplifier for gravitational wave detectors,” Optics Express, Vol. 15, No. 2, 2007. A single-pass, four-stage amplifier described in the Maik Frede et al. paper and diagrammed in FIG. 2, extracted only 3 W from an amplifier with a 1 W seed laser and 45 W of pump power, which yields an optical-to-optical efficiency of 6.7%. Even state of the art single-pass power amplifiers typically exhibit a low extraction efficiency or a high (40%-60%) optical conversion rate from a diode laser pumping light source. However, a typical diode end-pumped vanadate laser oscillator has 40%-60% optical-to-optical conversion efficiency.
A method of improving the energy extraction efficiency entails guiding the laser beam back through the gain material multiple times, thereby compounding the gain until the desired power amplification is achieved. A typical multi-pass amplifier produces much more gain than does a single-pass amplifier. Suitable applications for a multi-pass power amplifier include semiconductor device link processing (IR, green and UV tailored pulse), laser micromachining (picosecond pulse amplification), and via drilling (high-power IR, green, and UV laser). U.S. Pat. No. 5,546,222, of Plaessmann et al. describes several embodiments of a multi-pass light amplifier, four of which embodiments are presented in FIG. 3. The Plaessmann et al. patent demonstrates, using a Nd:YLF twelve-pass amplifier at 10 kHz, that 2.5 μJ of energy was amplified to 45 μJ with 1.6 W of pump energy focused in the amplifier gain medium. As is typical for traditional multi-pass configurations, a large gain, in this case, twenty-fold, is achieved at the expense of beam quality.
A number of patents describe multi-pass amplifiers, but all of them share the problem of laser beam displacement within the gain medium, which displacement has two inherent, serious drawbacks. The first is that the pumped region must be sufficiently large to contain all the laser modes in different passes; otherwise, the result is low efficiency mode matching between the laser and the pump. Second, a non-uniform pump distribution in the gain medium, such as the so-called “super Gaussian” mode, causes distortion in laser beam power distribution with each pass, ultimately resulting in degradation of laser beam quality. Therefore, similar to a laser cavity with thermal lensing, compensation optics are needed to optimize laser output with higher beam quality. In addition, these multi-pass amplifiers generally require a fairly complicated optical setup, possibly even specially shaped optical elements. More important, multi-pass laser beams normally share the same two or three optical elements, making it fairly difficult to control the influence of thermal lensing. This especially causes problems in high-power applications because each pass modifies the laser beam parameters.
U.S. Pat. No. 5,268,787 of McIntyre describes a method and an apparatus for multi-pass laser amplifiers but does not address thermal depolarization issues and unwanted lasing in the amplifier. It also fails to address how the gain material, the key component of a laser power amplifier, affects performance of the laser amplifier when pumped by a high power light source. In the case of YAG solid state lasers, high power pumping induces significant thermal birefringence, causing orthogonal polarization directions to exhibit different gain in such a setup. Thermally induced birefringence in YAG rods under strong optical pumping has been observed, reported, and analyzed in numerous articles. Q. Lu et al., “A novel approach for compensation of birefringence in cylindrical Nd:YAG rods,” Optical Quantum Electronics, Vol. 28, pp. 57-69, 1996, showed that 25% of optical power was lost through laser beam depolarization caused by thermal birefringence. Q. Lu et al. report that a carefully designed compensation method reduced the power loss to just 5%. Thus, it would seem that controlling and compensating for thermal birefringence in laser amplifiers is necessary and important.
U.S. Pat. No. 6,384,966 of Dymott addresses this power loss problem by rearranging optical components of a previous laser amplifier design to compensate for thermal birefringence, while passing the laser beam multiple times through the gain medium. For example, in the Dymott patent, a quarter-wave plate is placed between the gain medium and a first reflecting mirror. The Dymott patent specifies that the quarter-wave plate be oriented such that linearly polarized beam emerging from a Faraday rotator pass through the quarter-wave plate without undergoing any phase retardation. However, because of thermally induced birefringence, light passing once through the gain material generally becomes elliptically polarized. Upon two passes through the quarter-wave plate, the rotation direction of the elliptical polarization is reversed, and the thermally induced birefringence in the gain material is compensated.
The Dymott patent describes use of additional optical components in the design of the optical power amplifier to address other issues. For example, a 450 polarization rotator, or “Faraday rotator,” is needed in this amplifier to separate amplified light from incident seed light. But the Faraday rotators (reference numerals 2, 4, 23, and 73 in FIGS. 1-5 of the Dymott patent) are placed in a region where the laser beam spot size is difficult to control, potentially causing damage in the case of high-average-power and high-peak-power applications. Another example is the placement of a pair of concave and convex mirrors on either side of each laser crystal to construct an unstable cavity to eliminate undesired lasing action.
In addition, the strong thermal lens in high-power applications acts as a major lens in the amplifier, contributing to instability of the cavity. As is well-known, the degree of thermal lensing varies with PRF, cooling temperature, and pump power. Multi-pass power amplifiers described in the Dymott patent are fabricated from Nd:YAG, an isotropic gain medium that is subject to depolarization effects. The Dymott patent points out that gain materials may include Nd:YAG, Nd:YVO4, Nd:YLF, or Ti:sapphire to compensate for thermally induced birefringence, by design.